The present invention relates to tests for measuring ion concentrations in aqueous solutions. More particularly, the invention relates to a method for reducing sodium and potassium interferences in tests for other ions such as lithium.
Accurate measurement of ion concentrations in aqueous solutions such as blood is very important. For example, the toxic dose levels of lithium in blood are only slightly higher than the therapeutic levels used in psychiatric treatment.
Many different methods for determining the concentration or presence of ions in solution have been developed. These include ion-specific electrodes, liquid/liquid partitioning, fluorescence enhancement, chromophore labeled ionophores, and test strips. A general discussion of each of these methods is disclosed in U.S. Pat. No. 4,670,218 which is incorporated herein by reference. These methods make use of compounds and compositions called ionophores which are molecules capable of forming a complex with a particular ion, in some instances to the substantial exclusion of other ions.
U.S. Pat. No. 4,670,218 discloses in particular a test means which comprises a porous carrier matrix substantially uniformly incorporated with (a) a homogenous hydrophobic mixture containing three principal ingredients: (1) an ionophore capable of forming a complex with a specific ion to be determined, (2) a hydrophobic substance and (3) a reporter substance capable of interacting with the complex of the ionophore and ion to produce a detectable response; and (b) a buffering substance capable of providing a pH in the range of from about 5 to 10. The patent also teaches that interferant removal substances can be incorporated into the test means.
In use, an aqueous test sample is contacted with the test means. The presence and/or concentration of the specific ion to be determined in the test sample is then determined by observing any detectable response produced. The most common reporter substances are those which cause a color change to occur.
One of the problems encountered in measuring ion concentrations is interference from other ions. For example, many of the ionophores used to complex with lithium also complex with sodium. The high concentration of sodium in serum (about 140 mM) is a serious interferant in the test for lithium concentrations in serum (about 1 mM). Accordingly, it would be a significant advancement in the art to provide a method and apparatus for measuring lithium ion concentrations in which interference from other ions is reduced or eliminated. It would be a further advancement in the art to provide such a method and apparatus which were relatively simple to perform and use. Such a method and apparatus are disclosed and claimed herein.